The present invention relates to a heat activated epoxy adhesive and its use in a foam insert that is bonded to a metal body. Structural foam inserts (SFIs) have been developed to reinforce structures within motor vehicles to add strength and stiffness to the vehicle at the site of insertion. Acoustical foam inserts (AFI) have been developed to reduce the impact of noise and vibration on vehicle passages. For example, an SFI coated with an uncured expandable epoxy adhesive is secured to the B-pillar cavity of an automobile. An AFI is typically placed in a hollow part of an automobile for the purpose of preventing the transmission of noise and vibration throughout the hollow cavity. AFI may comprise a foam coated with an expandable adhesive. The automobile body is then subjected to electrodeposition coating (e-coating) and bake, whereupon the epoxy adhesive expands through heat activation to form a bond between the foam insert and the sheet metal. However, one problem associated with expandable adhesives of the prior art is their propensity to crosslink before they expand. This premature crosslinking results in ineffective wet-out of the vehicular substrate and concomitant weaker bonding. Furthermore, solving the problem of premature crosslinking—for example, by reducing or eliminating catalyst—creates another problem, namely, the formation of a cured expanded polymer with acceptable adhesion at the expense of large voids (number weighted mean diameter of >2000 μm), the formation of which decreases durability and results in reduction in mechanical properties.
It would therefore be an advance in the art of foam inserts to provide an expandable adhesive for the insert that efficiently wets out a substrate before crosslinking occurs, thereby creating optimal chemical bonding and enhanced durability, but which produces small voids, thereby resulting in enhanced mechanical properties.
The adhesive of the present invention can expand up to 350 percent. To insure the cavity is filled, expansion is limited by the available space in the cavity. The adhesive present invention creates an equalizing pressure which helps align the part in the cavity.